With an increase in integration density of semiconductor elements and an increase in packaging density of electronic components, the number of input/output terminals of the semiconductor elements and the electronic devices using the same is increasing. For example, in a semiconductor element to be flip-chip mounted, the pitch between connection terminals is reduced and further the area of the connection terminals is also reduced.
In order to achieve high-speed operation, severe demands are imposed on current semiconductor elements in which high-speed operation is desired. For example, in a current high-speed semiconductor element, such as a large scale integrated circuit (LSI), so-called low-K materials, such as porous silica, are used as an interlayer insulation film in order to reduce the parasitic capacitance between wiring patterns. However, the low-K materials have problems in that the materials generally have a low density corresponding to a low dielectric constant, and therefore the materials are mechanically vulnerable and are easily damaged due to thermal distortion during joining. For example, porous silica has an elastic modulus of 4 to 8 GPa, and the mechanical strength thereof is lower than that of conventional interlayer insulation materials, such as a silicon oxide film.
Under such a situation, the high-speed semiconductor elements containing the low-K materials are desired to reduce thermal distortion of a substrate during joining by joining the connection terminals at a low temperature when manufacturing a semiconductor device by flip-chip mounting of a semiconductor chip. However, a generally-used lead-free solder for joining the connection terminals is used at a temperature of 217° C. or higher for joining, and is not suitable for joining at such a low temperature. Under such a situation, in mounting of the high-speed semiconductor elements containing the low-K materials, an eutectic Sn(tin)-Bi (bismuth) solder having a melting point of 139° C. or a solder in which a little amount of elements, such as Ag, Cu, and Sb, is added to Sn—Bi for the purpose of improving the mechanical characteristics, such as ductility, is used as a solder material capable of reducing thermal stress in many cases.
As described above, the eutectic Sn—Bi solder has a melting point of 139° C. and may be mounted at a temperature lower by about 80° C. than, for example, an Sn—Ag—Cu solder (Melting point of 217° C.) which is a conventional lead-free solder.
However, there is a demand in an actual electronic device such that, in order to secure the reliability of the electronic device, the electronic device is subjected to a temperature cycle test or a high temperature exposure test at an environmental temperature of about 150° C. considering the actual environment. However, when such a test is performed, the environmental temperature (150° C.) of the test exceeds the melting point (139° C.) of the Sn—Bi solder, which may cause a problem of re-melting of a junction portion or the like.
In a semiconductor device or an electronic device having a configuration in which a large number of circuit boards and semiconductor chips are stacked, a problem may arise such that a portion, which is previously joined by reflowing solder bumps, melts in the reflow of solder bumps to be performed later in the semiconductor device or the electronic device.
Examples of the above-described related art is disclosed in Kenichi YASAKA, Yasuhisa OHTAKE, et al., “Microstructural Changes in Micro-joins between Sn-58Bi Solders and Copper by Electro-migration” ICEP 2010 Proceedings FA2-1, pp. 475-478, and OHTAKE et al., “Electro-migration in Microjoints between Sn—Bi Solders and Cu”, 16th Symposium on Microjoining and Assembly Technology in Electronics, Feb. 2-3, 2010, Yokohama, pp 157-160.